1. Technical Field
This invention in general relates to semiconductor circuits. More specifically, this invention relates to circuits for driving columns of an organic light emitting diode (OLED) displays.
2. Description of the Related Art
Recently, much progress has been made in organic light emitting diode (OLED). An OLED display is made up of rows and column electrodes for selectively activating the OLED at each intersection. The row and column electrodes are driven by a row driver and a column driver, respectively. The row electrodes are scanned in sequence to refresh the display image.
As the OLED display becomes larger with an increased number of rows, the row scan frequency should increase, which reduces the time during which electric current is supplied for each OLED. In order to supply enough current for a shorter duration, a higher voltage needs to be supplied. The lifetime of an OLED device, however, deteriorates as current and voltage levels increase. In addition, in order to make the design withstand higher voltage application, a design rule of larger feature size as well as an expensive processing technology is needed.
In order to lengthen the lifetime, many OLED displays use a dual scan scheme. FIG. 1 shows the dual scan scheme where an OLED panel 10 is divided into an upper panel 11 and a lower panel 12. Although there is one row driver 13, there are two column drivers 14 and 15, each driver responsible for each half panel.
There are, however, several problems with the conventional dual-scan scheme. First, there is a problem with uniformity in brightness. Because there is a chip-to-chip variation in the output current, the brightness of the upper half panel is not the same as that of the lower half panel.
Second, there is a problem with the part count. Since the bright-current relationship is not the same for Red, Green, and Blue (RGB), it is preferable to independently control the different levels of current supply for R, G and B at a given brightness. If a single design is used for both upper and lower panel, when two column drivers are attached to the glass, the chip should be rotated by 180 degrees. This would not then allow individual control of the current level unless the second chip is prepared using a mirror image, which, in turn, would increase the part count.
Therefore, there is a need for a new column driver that can drive a dual-scanning OLED display with uniform brightness without increasing the part count.